A presently used method includes providing a milk sample from the process plant in an open sample container or cup from which dissolved gases may escape, passing part of the degassed milk sample from the container through a measurement branch into a measurement cuvette, performing the test and passing the tested milk sample to a waste outlet.
The presently used test instrument includes generally a flow system, an IR spectrophotometer, and a computer comprising a PC with hard-disc, floppy disc drive, monitor and keyboard.
Instead of the above mentioned method it would be preferable to perform the test on-line and in-line in the process plant, and preferably in such a way that it also would be possible to let the tested milk sample be returned into the process line, to avoid the waste.
U.S. Pat. No. 5,137,738 discloses a system and a method for controlling the butterfat content of milk. The entire product stream is monitored continuously by the use of optical density sensors. The preferred sensors have stainless steel bodies and housings med Pyrex windows and mount directly on the product output lines and operate at full flow and pressure. U.S. Pat. No. 4,144,804 discloses a photoelectric monitoring system for continuously measuring the butterfat content of a sample of homogenized milk as the latter flows continuously through the processing system under the pressure of the homogenizer in the system. The known on-line sensors for milk products do not apply spectrometric analysis of the content. On-line spectrum measurement for determining a property of a product is known, e.g. as described in U.S. Pat. No. 5,452,232 for a hydrocarbon product. GB-A-2 104 681 describes an apparatus for the continuous investigation of chemical reactions by infrared absorption by use of an IR spectrophotometer having a through flow cell through which flows a continuous sample stream branched off from the reaction container. U.S. Pat. No. 4,910,403 describes a flow cell utilized on-line in the analysis of molten polymer. The flow cell includes diamond windows for passing mid to far infrared radiation through the molten polymer flowing through the flow cell.
Testing milk in-line in a process plant in a dairy presents several problems. One problem is due to dissolved air, which is normally present in the raw milk. If air bubbles are released and enter the measuring cuvette, the measurement result will obviously not show a correct analysis of the milk product itself. A further problem is that milk includes several components and specifically the fat globules can give reason to failures. According to the known art the temperature of the milk sample should be raised to about 35-42.degree. C., and preferably homogenized to make reliable, reproducible measurements possible in an IR cuvette. Obviously, the milk in the process plant is generally kept at a low temperature to avoid the milk to be spoiled by some unwanted reactions, such as growing bacteria's.
A further problem is that thin layers of milk tend to adhere to the IR-windows of the cuvette. The measurements may be seriously deteriorated due to such milk coatings. Therefore, an IR cuvette needs regular thorough cleaning.
An in-line and on-line system must be able to measure reliably and normally without needing any regular calibrations and adjusting. Spectrometric measurements require the utmost stability of the components of the optical system. It is very important that the cuvette is extremely stable and not liable to suffer from wear. In fact the cuvette is highly exposed to wear as the liquid product and rinsing and cleaning solutions pass through the cuvette under high pressure and flow rates.
It has therefore until now not been possible to perform the desired testing procedures on milk in an in-line system. To the applicants best knowledge there does not on the market exist any reliable apparatus able to perform accurate and reliable in-line determinations of the quantities of the components in raw milk or in a milk product, e.g. fat, protein, lactose, urea and casein.